1. Field of the Invention
This invention relates to a process for catalytic cracking of petroleum oil. More particularly, it relates to the application of appropriate control and monitoring conditions to a fluid catalytic cracking (“FCC”) process operating at relatively high severity conditions, which includes cracking of petroleum oil to obtain a maximum yield of a light olefin such as propylene, thereby maximizing propylene production per unit of hydrocarbon feed.
2. Description of the Prior Art
Although steam pyrolysis is widely practiced in process of cracking petroleum oil, this process is energy intensive, not very selective, produces coke and releases significant amounts of carbon dioxide into the air. Chemical manufacturers have long recognized a need for an alternative hydrocarbon cracking process. One alternative to steam pyrolysis process is a catalytic cracking process.
In a typical catalytic cracking unit, petroleum-derived hydrocarbons are catalytically cracked with a catalyst to obtain gasoline as the main product, a small amount of LPG, and cracked gas oil. Coke deposited on the catalyst is then burnt away with air to recycle the regenerated catalyst for reuse
In a typical FCC process light olefin selectivity may be increased by increasing the reaction temperature which causes an increase in the contribution of thermal cracking and, thus, leads to increased formation of lighter products. For instance, in a specific type of FCC process, referred to as a Deep Catalytic Cracking (“DCC”) process, higher temperatures and increased amounts of steam are used. However, thermal cracking in the DCC process is not very selective and produces large amounts of products of relatively little value, such as hydrogen, methane, ethane, and ethylene, in the “wet gas” (which contains H2 and C1-C4 products). Wet gas compression often limits refinery operation.
Another way to increase light olefin selectively is to include an olefin-selective zeolite-containing additive such as a ZSM-5-containing additive in the process. Conventional additives such as ZSM-5 selectively convert primary cracking products (e.g., gasoline olefins) to C3 and C4 olefins. Improvement of the activity or the selectivity with phosphorus is known to increase the effectiveness of ZSM-5. However, the additives may dilute the catalyst inventory and decrease bottoms conversion.
The known FCC methods cannot produce sufficient light-fraction olefins selectively. For example, the high-temperature cracking reaction will result in a concurrent thermal cracking of petroleum oils, thereby increasing the yield of dry gases from feedstock oils.
The reaction of feed oil with a catalyst during short contact time cause a decrease of conversion of light-fraction olefins to light-fraction paraffins due to its inhibition of a hydrogen transfer reaction. During the short contact time reactions, the conversion of petroleum oils to light-fraction oils are not greatly increased. Furthermore, the use of pentasil-type zeolites only enhance the yield of light-fraction hydrocarbons by excessive cracking of the gasoline, once it is produced. Therefore, it is difficult to produce light-fraction olefins from heavy fraction oils in a high yield by using either of these known techniques. Therefore, there is a need to use a new method to optimize production conditions where the reaction time is optimized with a view to produce certain desired end products.
Further, in general, the difficulty in FCC is that the reactor and stripper temperatures should be maximized where as the regenerator temperature is to be minimized. Controlling temperature in this manner, does not effectively occur in conventionally heat balanced operations because any increase in the reactor temperature essentially leads to an increase in the regenerator temperature also. Therefore, a need exists for appropriate control systems that allow appropriate heat-balances in a FCC unit.
Additionally, in typical FCC processes, the catalyst is manually augmented during the refining process to control the emissions and product mix. In other words, there is no systematic feedback mechanism for optimizing such a manual process.
Due to the uncertain chemical make-up of the feedstock entering the FCC system, both the emissions and the product mix may vary or drift from process targets during the course of refining. As a result, system operators must closely monitor system outputs and to be constantly available to make manual adjustments to the catalyst injection schedule as needed. Operating in this manner causes a significant challenge if the system operates under severe conditions. Thus, it would be beneficial to be able to remotely monitor and control the overall process and allow the process model to advise adjustments through catalyst injections to the system outputs while reducing the reliance on human interactions such as monitoring and manual changes to the catalyst injection schedule.
Moreover, the process variables are not necessarily optimized in existing FCC processes for maximizing conversion of propylene, especially if the FCC operates at a severe mode. An optimum conversion level corresponding to a given feed rate, feed quality, set of processing objectives and catalyst at other unit constraints (e.g., wet gas compressor capacity, fractionation capacity. air blower capacity, reactor temperature, regenerator temperature, catalyst circulation). Therefore, the operator must manually adjust several variables at the same time, making the task nearly impossible because of the lack of suitable automation process equipment that can be readily used to optimize such performance.